1. Field of the Invention
This invention relates to a compound semiconductor solar cell and a production method thereof. More particularly, it relates to a compound semiconductor solar cell having a p-n junction and a production method thereof.
2. Description of the Related Art
FIGS. 6(a) and 6(b) show a compound semiconductor solar cell having a p-n junction light absorbing layer according to the prior art. FIG. 6(a) is a front view of the solar cell of compound semiconductor and FIG. 6(b) is a longitudinal sectional view. This compound semiconductor solar cell (hereinafter called merely the "solar cell" in some cases) includes a molybdenum layer 102 on a glass substrate 100. A p-type semiconductor layer 104 and an n-type semiconductor layer 106 are formed serially on this molybdenum layer 102. A transparent electrode 108 is formed on the n-type semiconductor layer 106. A comb-shaped electrode 110 is formed on the transparent electrode 108. In this comb-shaped electrode 110, an electrode is formed in a branched shape (a comb shaped) as shown in FIG. 6(a).
The solar cell shown in FIGS. 6(a) and 6(b) can be produced by the method that is shown in FIGS. 7(a) to 7(d). First, an electrode film comprising the molybdenum layer 102 is formed on one of the surfaces of the glass substrate 100 by evaporation or sputtering. Next, an indium layer 103 is evaporated at room temperature. A copper layer 105 is further evaporated on the indium layer 103 at room temperature (process step shown in FIG. 7(a)).
The metallic film comprising the indium layer 103 and the copper layer 105 are heat-treated in a hydrogen sulfide atmosphere, for sulfurization treatment, to convert the metallic film to a p-type semiconductor layer 104 of CuInS.sub.2. A KCN treatment for etching the surface of this p-type semiconductor layer 104 with a KCN solution containing 5 to 10% by weight of KCN is conducted. In this way, impurities formed in the p-type semiconductor layer 104, such as Cu.sub.x S.sub.y (sulfide), are removed and the characteristics of the p-type semiconductor layer 104 are optimized and stabilized (step shown in FIG. (b)).
An n-type semiconductor layer 106 is further formed on the p-type semiconductor layer 104 by a chemical bath deposition method (step shown in FIG. 7(c)). A transparent electrode 108 made of Al or In.sub.2 O.sub.3 is formed on the n-type semiconductor layer 106 by sputtering (step shown in FIG. 7(d).
A comb-shaped electrode 110 made of aluminum is formed on the transparent electrode 108. Furthermore electrode terminals (not shown) are formed on the molybdenum layer 102 in the solar cell shown in FIGS. 6(a) and 6(b).
In the solar cell shown in FIGS. 6(a) and 6(b), crystallinity inside the p-type semiconductor layer 104 can be improved by increasing as much as possible the Cu/In atomic ratio of copper (Cu) to indium (In) forming the p-type semiconductor layer 104 before the KCN treatment (the term "Cu/In atomic ratio" will hereinafter mean the Cu/In atomic ratio of copper (Cu) to indium (In) forming the p-type semiconductor layer 104 before the KCN treatment) and/or by maximizing the thickness of the p-type semiconductor layer 104. As a result, power generation efficiency of the solar cell can be improved.
In existing solar cells of this kind, however, the Cu/In atomic ratio of the p-type semiconductor layer 104 before the KCN treatment is at most about 1.6, from the aspect of the production yield of the solar cells finally obtained, because, if the Cu/In atomic ratio is increased beyond 1.6, the p-type semiconductor layer 104 becomes more likely to peel from the electrode film 102 during the KCN treatment.
The upper limit of the P-type semiconductor layer 104 is also about 2 .mu.m. When the thickness of the metallic film comprising the indium layer 103 and the copper layer 105 is increased so as to form a p-type semiconductor layer 104 which is more than 2 .mu.m thick, peeling of the p-type semiconductor layer 104 occurs during the KCN treatment.
For these reasons, in the production methods according to the prior art it has been extremely difficult to improve power generation efficiency of the solar cell by improving crystallinity inside the p-type semiconductor layer 104.
In a compound semiconductor solar cell including a p-type semiconductor layer formed mainly of copper (Cu) and indium (In), with gallium (Ga) whenever necessary, and having a p-n junction, it is therefore a main object of the present invention to provide a compound semiconductor solar cell that has a higher Cu/In atomic ratio of copper (Cu) to indium (In) before a KCN treatment than that of the prior art cells, and that has improved crystallinity inside the p-type semiconductor layer, and to provide a production method for such a solar cell.